The hard knocks that come with playing an instrument frequently will loosen glue joints. Also some older instruments were made with poorly formulated glues that weaken or embrittle with age. Some loose joints are subtle like a loosened brace behind the face; nothing shows but you might hear a buzz or rattle when playing it. In this guitar virtually every brace had to be reglued. Some had fallen loose within the instrument!
More serious joints might show as a separating back, a loose neck or a crack in the tuning head. All of these can be made to disappear with skill and patience.
The long brace visible was an invention of my own allowing me to adjust the angle of the neck (and therefore the string height) before final regluing of the back.